1. Field of the Invention
The invention relates to a drive network for an ultrasonic probe and, in particular, to a drive network having an arrangement therein adapted to search for the resonant frequency of the probe in an efficient ordered manner.
2. Description of the Prior Art
Ultrasonic vibratory probes such as that sold by, among others, Heat Systems-Ultrasonics Inc., Farmingdale, N.Y. operate most efficiently when the frequency of the excitation energy applied to the probe is substantially equal to the resonant frequency of the probe. Once a probe is excited at its resonant frequency, however, variations in the probe or in the loading to which the probe is exposed will cause the resonant frequency of the system to change. Known in the art are various arrangements whereby the frequency of vibration of the probe is monitored during its operational cycle in order to continuously track the resonant frequency of the system of which the probe is a part so that maximum energy transfer may occur. Exemplary of such systems are those described in U.S. Pat. Nos. 4,468,581 (Okada et al.); 4,363,242 (Heyman); 4,302,728 (Nakamura); 4,277,710 (Harwood et al.); 4,175,242 (Kleinochmidt); 3,931,533 (Raso et al.); 4,578,650 (Watson); and German Patent No. 2,721,225 (Siemens).
Upon startup of the probe it is the usual practice to select an excitation frequency, couple the same to the probe and observe response thereof. U.S. Pat. No. 4,445,064 (Bullis), assigned to the assignee of the present invention, is an example of a system having limited tracking and no search capability. If the probe vibrates it is assumed that the selected excitation frequency is sufficiently close to the resonant frequency of the probe so that the transfer of energy can occur. However, such a mode of initiating the operation of the probe is not believed advantageous.
U.S. Pat. Nos. 4,562,413 and 4,275,363 (both Mishiro) disclose complex differential current arrangements whereby the resonant frequency of the probe may be systematically searched. Neither of these arrangements utilize a phase locked loop network in conducting the search for the resonant frequency.
As noted earlier, since the resonant frequency of the probe changes with use or after repair, it would be advantageous to provide an arrangement whereby, upon initiation of the operation of the probe, the resonant frequency thereof may be quickly and accurately established. Accordingly, in view of the foregoing, it is believed advantageous to provide a network operable on the initial excitation of the ultrasonic probe for efficiently seeking the resonant frequency of the system.